U.S. patent application number 15/360262 was filed with the patent office on 2018-05-24 for regenerative braking downshift control using predictive information.
The applicant listed for this patent is Ford Global Technologies, LLC. Invention is credited to Ming Lang KUANG, Bernard D. NEFCY, Yanan ZHAO.
Application Number | 20180141557 15/360262 |
Document ID | / |
Family ID | 62069047 |
Filed Date | 2018-05-24 |
United States Patent
Application |
20180141557 |
Kind Code |
A1 |
NEFCY; Bernard D. ; et
al. |
May 24, 2018 |
REGENERATIVE BRAKING DOWNSHIFT CONTROL USING PREDICTIVE
INFORMATION
Abstract
A vehicle may include an engine selectively coupled to a motor
and a transmission. The vehicle may include a controller configured
to, in response to actuation of a brake pedal, command the
transmission to downshift during a regenerative braking event based
on a regenerative braking downshift torque. The regenerative
braking downshift torque may be determined from a predicted brake
pedal input rate. The predicted brake pedal input rate may be based
on road grade, vehicle headway range and a driver history. The
predicted brake pedal input rate may be classified as Low, Medium,
or High. The regenerative braking downshift torque may also be
determined from a predicted brake torque rate that is based on a
predicted deceleration rate of the vehicle, a vehicle speed
prediction and a road grade prediction within a future time
interval that begins upon actuation of the brake pedal.
Inventors: |
NEFCY; Bernard D.; (Novi,
MI) ; ZHAO; Yanan; (Ann Arbor, MI) ; KUANG;
Ming Lang; (Canton, MI) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Ford Global Technologies, LLC |
Dearborn |
MI |
US |
|
|
Family ID: |
62069047 |
Appl. No.: |
15/360262 |
Filed: |
November 23, 2016 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
B60W 2710/083 20130101;
B60W 2540/30 20130101; B60W 2552/15 20200201; B60W 2554/801
20200201; B60W 2554/80 20200201; Y02T 10/6221 20130101; B60W 20/30
20130101; B60K 2006/4825 20130101; B60W 2420/50 20130101; Y02T
10/62 20130101; B60W 20/00 20130101; Y02T 10/6252 20130101; B60W
2556/50 20200201; B60W 2540/12 20130101; Y10S 903/93 20130101; Y10S
903/945 20130101; B60W 10/11 20130101; B60Y 2300/18125 20130101;
B60K 6/48 20130101; B60W 30/18127 20130101; B60W 10/08 20130101;
B60W 50/0097 20130101; B60W 2710/1005 20130101 |
International
Class: |
B60W 30/18 20060101
B60W030/18; B60W 10/08 20060101 B60W010/08; B60W 10/11 20060101
B60W010/11; B60W 20/00 20060101 B60W020/00 |
Claims
1. A vehicle, comprising: an engine selectively coupled to a motor
and a transmission; and a controller configured to, in response to
actuation of a brake pedal, command the transmission to downshift
during a regenerative braking event based on a regenerative braking
downshift torque determined from a predicted brake pedal input
rate.
2. The vehicle of claim 1, wherein the predicted brake pedal input
rate is based on an instantaneous road grade calculated upon
actuation of the brake pedal and an average road grade estimated
over a future time interval that begins with actuation of the brake
pedal.
3. The vehicle of claim 2, wherein the instantaneous road grade is
determined from at least one of geographical information systems
and global positioning systems.
4. The vehicle of claim 1, wherein the predicted brake pedal input
rate is based on a driver history.
5. The vehicle of claim 1, wherein the predicted brake pedal input
rate is based on a headway range and a rate of change thereof.
6. The vehicle of claim 5, wherein the headway range is based on a
distance to a preceding car measured using electromagnetic
waves.
7. The vehicle of claim 1, wherein the predicted brake pedal input
rate is classified as one of low, medium, and high.
8. The vehicle of claim 1, wherein the regenerative braking
downshift torque is further determined from a minimum regenerative
torque that is based on a minimum motor torque and a threshold
value associated with a specified margin of operation of the
motor.
9. The vehicle of claim 1, wherein the regenerative braking
downshift torque is further determined from a transmission shift
time that is based on an amount of time between execution of a
transmission downshift and a change in a torque ratio resulting
from downshifting the transmission during the regenerative braking
event.
10. A vehicle, comprising: an engine selectively coupled to a motor
and a transmission; and a controller configured to, in response to
actuation of a brake pedal, command the transmission to downshift
during a regenerative braking event based on a regenerative braking
downshift torque determined from a predicted brake torque rate.
11. The vehicle of claim 10, wherein the predicted brake torque
rate is based on a predicted deceleration rate of the vehicle over
a future time interval that begins upon actuation of the brake
pedal.
12. The vehicle of claim 10, wherein the predicted brake torque
rate is based on a vehicle speed prediction and a road grade
prediction within a future time interval that begins upon actuation
of the brake pedal.
13. The vehicle of claim 10, wherein the regenerative braking
downshift torque is further determined from a minimum regenerative
torque that is based on a minimum motor torque and a threshold
value associated with a specified margin of operation of the
motor.
14. The vehicle of claim 10, wherein the regenerative braking
downshift torque is further determined from a transmission shift
time that is based on an amount of time between execution of a
transmission downshift and a change in a torque ratio resulting
from downshifting the transmission during the regenerative braking
event.
15. A method comprising: in response to a brake pedal actuation,
commanding a transmission of a vehicle to downshift during a
regenerative braking event based on a regenerative braking
downshift torque determined from one of a predicted brake pedal
input rate and a predicted brake torque rate.
16. The method of claim 15, wherein the predicted brake pedal input
rate is based on a difference between an instantaneous road grade
and an average road grade.
17. The method of claim 15, wherein the predicted brake pedal input
rate is based on a driver history.
18. The method of claim 15, wherein the predicted brake pedal input
rate is based on a headway range and a rate of change thereof.
19. The method of claim 15, wherein the predicted brake torque rate
is based on a vehicle speed prediction and a road grade prediction
within a future time interval that begins in response to the brake
pedal actuation.
20. The method of claim 15, wherein the predicted brake torque rate
is based on a predicted deceleration rate of the vehicle over a
future time interval that begins in response to the brake pedal
actuation.
Description
TECHNICAL FIELD
[0001] The present disclosure relates to an automatic transmission
downshifting strategy to increase regenerative braking
efficiency.
BACKGROUND
[0002] Regenerative braking is used on many hybrid and electric
vehicles to generate electricity during braking events to increase
fuel economy. During these events, kinetic energy of the vehicle is
converted to electricity for charging a high voltage battery using
an electric machine as a brake and a generator. Since regenerative
braking efficiency decreases at lower speeds, an automatic
transmission of the vehicle may be downshifted to increase the
electric machine speed and increase the available torque and
efficiency. It is desirable to provide systems and methods for
scheduling the first downshift of the automatic transmission at a
proper time during the brake application such that the electric
machine can deliver its maximum power to increase regenerative
braking efficiency and fuel economy.
SUMMARY
[0003] According to embodiments of the present disclosure, systems
and methods for controlling a transmission downshift during a
regenerative braking event to increase regenerative braking
efficiency and fuel economy are disclosed. In particular, the
timing of the first regenerative braking downshift is adjusted
based on the upcoming or predicted brake torque rate and/or brake
pedal input rate.
[0004] In one embodiment, a vehicle is disclosed having an engine
selectively coupled to a motor and a transmission. The vehicle
includes a controller configured to, in response to actuation of a
brake pedal, command the transmission to downshift during a
regenerative braking event based on a regenerative braking
downshift torque. The regenerative braking downshift torque is
determined from a predicted brake pedal input rate. The predicted
brake pedal input rate may be based on an instantaneous road grade
calculated upon actuation of the brake pedal and an average road
grade estimated over a future time interval that begins with
actuation of the brake pedal. The predicted brake pedal input rate
may also be based on a driver history. The predicted brake pedal
input rate may further be based on a headway range and a rate of
change thereof. The predicted brake pedal input rate may be
classified as one of Low, Medium, and High. The regenerative
braking downshift torque may also be determined from a minimum
regenerative torque that is based on a minimum motor torque and a
threshold value associated with a specified margin of operation of
the motor.
[0005] In another embodiment, a vehicle is disclosed having an
engine selectively coupled to a motor and a transmission. The
vehicle includes a controller configured to, in response to
actuation of a brake pedal, command the transmission to downshift
during a regenerative braking event based on a regenerative braking
downshift torque determined from a predicted brake torque rate. The
predicted brake torque rate may be based on a predicted
deceleration rate of the vehicle over a future time interval that
begins upon actuation of the brake pedal. The predicted brake
torque rate may also be based on a vehicle speed prediction and a
road grade prediction within a future time interval that begins
upon actuation of the brake pedal. The regenerative braking
downshift torque may further be determined from a transmission
shift time that is based on an amount of time between execution of
a transmission downshift and a change in a torque ratio resulting
from downshifting the transmission during the regenerative braking
event.
[0006] In yet another embodiment, a method is disclosed that
includes, in response to a brake pedal actuation, commanding a
transmission of a vehicle to downshift during a regenerative
braking event based on a regenerative braking downshift torque
determined from one of a predicted brake pedal rate and a predicted
brake torque rate. The predicted brake pedal rate may be based on a
difference between an instantaneous road grade and an average road
grade.
[0007] Various embodiments may provide one or more advantages. For
example, regenerative braking downshift control according to
various embodiments adjusts the first regenerative braking
downshift timing based on the upcoming or predicted brake rate.
Thus, the faster the brake is applied, the earlier the first
regenerative braking downshift will occur. This helps to increase
regenerative braking efficiency and fuel economy. The above
advantages and other advantages and features of various embodiments
of the claimed subject matter may be recognized by those of
ordinary skill in the art based on the representative embodiments
described and illustrated.
BRIEF DESCRIPTION OF THE DRAWINGS
[0008] FIG. 1 is a schematic illustration of a hybrid vehicle
according to one or more embodiments of the present disclosure;
[0009] FIG. 2 is a flowchart illustrating a method for classifying
a brake pedal input rate using road grade predictive information
according to one or more embodiments of the present disclosure;
[0010] FIG. 3 is a flowchart illustrating a method for classifying
a brake pedal input rate using headway range predictive information
according to one or more embodiments of the present disclosure;
[0011] FIG. 4 is a flowchart illustrating a method for estimating
brake torque rate using predictive information according to one or
more embodiments of the present disclosure;
[0012] FIG. 5 is a flowchart illustrating a method for determining
a regenerative braking downshift torque according to one or more
embodiments of the present disclosure; and
[0013] FIG. 6 is a graphical illustration of the effects of shift
timing on motor torque according to one or more embodiments of the
present disclosure.
DETAILED DESCRIPTION
[0014] Embodiments of the present disclosure are described herein.
It is to be understood, however, that the disclosed embodiments are
merely examples and other embodiments can take various and
alternative forms. The figures are not necessarily to scale; some
features could be exaggerated or minimized to show details of
particular components. Therefore, specific structural and
functional details disclosed herein are not to be interpreted as
limiting, but merely as a representative basis for teaching one
skilled in the art to variously employ the embodiments. As those of
ordinary skill in the art will understand, various features
illustrated and described with reference to any one of the figures
can be combined with features illustrated in one or more other
figures to produce embodiments that are not explicitly illustrated
or described. The combinations of features illustrated provide
representative embodiments for typical applications. Various
combinations and modifications of the features consistent with the
teachings of this disclosure, however, could be desired for
particular applications or implementations.
[0015] Referring to FIG. 1, a schematic diagram of a hybrid
electric vehicle (HEV) 10 is illustrated according to an embodiment
of the present disclosure. FIG. 1 illustrates representative
relationships among the components. Physical placement and
orientation of the components within the vehicle may vary. The HEV
10 includes a powertrain 12. The powertrain 12 includes an engine
14 that drives a transmission 16, which may be referred to as a
modular hybrid transmission (MHT). As will be described in further
detail below, transmission 16 includes an electric machine such as
an electric motor/generator (M/G) 18, an associated traction
battery 20, a torque converter 22, and a multiple step-ratio
automatic transmission, or gearbox 24. The engine 14, M/G 18,
torque converter 22, and the automatic transmission 16 are
connected sequentially in series, as illustrated in FIG. 1.
[0016] The engine 14 and the M/G 18 are both drive sources for the
HEV 10. The engine 14 generally represents a power source that may
include an internal combustion engine such as a gasoline, diesel,
or natural gas powered engine, or a fuel cell. The engine 14
generates an engine power and corresponding engine torque that is
supplied to the M/G 18 when a disconnect clutch 26 between the
engine 14 and the M/G 18 is at least partially engaged. The M/G 18
may be implemented by any one of a plurality of types of electric
machines. For example, M/G 18 may be a permanent magnet synchronous
motor. Power electronics condition direct current (DC) power
provided by the battery 20 to the requirements of the M/G 18, as
will be described below. For example, power electronics may provide
three phase alternating current (AC) to the M/G 18.
[0017] When the disconnect clutch 26 is at least partially engaged,
power flow from the engine 14 to the M/G 18 or from the M/G 18 to
the engine 14 is possible. For example, the disconnect clutch 26
may be engaged and M/G 18 may operate as a generator to convert
rotational energy provided by a crankshaft 28 and M/G shaft 30 into
electrical energy to be stored in the battery 20. The disconnect
clutch 26 can also be disengaged to isolate the engine 14 from the
remainder of the powertrain 12 such that the M/G 18 can act as the
sole drive source for the HEV 10. Shaft 30 extends through the M/G
18. The M/G 18 is continuously drivably connected to the shaft 30,
whereas the engine 14 is drivably connected to the shaft 30 only
when the disconnect clutch 26 is at least partially engaged.
[0018] A separate starter motor 31 can be selectively engaged with
the engine 14 to rotate the engine to allow combustion to begin.
Once the engine is started, the starter motor 31 can be disengaged
from the engine via, for example, a clutch (not shown) between the
starter motor 31 and the engine 14. In one embodiment, the engine
14 is started by the starter motor 31 while the disconnect clutch
26 is open, keeping the engine disconnected with the M/G 18. Once
the engine has started and is brought up to speed with the M/G 18,
the disconnect clutch 26 can couple the engine to the M/G to allow
the engine to provide drive torque.
[0019] In another embodiment, the starter motor 31 is not provided
and, instead, the engine 14 is started by the M/G 18. To do so, the
disconnect clutch 26 partially engages to transfer torque from the
M/G 18 to the engine 14. The M/G 18 may be required to ramp up in
torque to fulfill driver demands while also starting the engine 14.
The disconnect clutch 26 can then be fully engaged once the engine
speed is brought up to the speed of the M/G.
[0020] The M/G 18 is connected to the torque converter 22 via shaft
30. The torque converter 22 is therefore connected to the engine 14
when the disconnect clutch 26 is at least partially engaged. The
torque converter 22 includes an impeller fixed to M/G shaft 30 and
a turbine fixed to a transmission input shaft 32. The torque
converter 22 thus provides a hydraulic coupling between shaft 30
and transmission input shaft 32. The torque converter 22 transmits
power from the impeller to the turbine when the impeller rotates
faster than the turbine. The magnitude of the turbine torque and
impeller torque generally depend upon the relative speeds. When the
ratio of impeller speed to turbine speed is sufficiently high, the
turbine torque is a multiple of the impeller torque. A torque
converter bypass clutch 34 may also be provided that, when engaged,
frictionally or mechanically couples the impeller and the turbine
of the torque converter 22, permitting more efficient power
transfer. The torque converter bypass clutch 34 may be operated as
a launch clutch to provide smooth vehicle launch. Alternatively, or
in combination, a launch clutch similar to disconnect clutch 26 may
be provided between the M/G 18 and gearbox 24 for applications that
do not include a torque converter 22 or a torque converter bypass
clutch 34. In some applications, disconnect clutch 26 is generally
referred to as an upstream clutch and launch clutch 34 (which may
be a torque converter bypass clutch) is generally referred to as a
downstream clutch.
[0021] The gearbox 24 may include gear sets (not shown) that are
selectively placed in different gear ratios by selective engagement
of friction elements such as clutches and brakes (not shown) to
establish the desired multiple discrete or step drive ratios. The
friction elements are controllable through a shift schedule that
connects and disconnects certain elements of the gear sets to
control the ratio between a transmission output shaft 36 and the
transmission input shaft 32. The gearbox 24 is automatically
shifted from one ratio to another based on various vehicle and
ambient operating conditions by an associated controller, such as a
powertrain control unit (PCU). The gearbox 24 then provides
powertrain output torque to output shaft 36.
[0022] It should be understood that the hydraulically controlled
gearbox 24 used with a torque converter 22 is but one example of a
gearbox or transmission arrangement; any multiple ratio gearbox
that accepts input torque(s) from an engine and/or a motor and then
provides torque to an output shaft at the different ratios is
acceptable for use with embodiments of the present disclosure. For
example, gearbox 24 may be implemented by an automated mechanical
(or manual) transmission (AMT) that includes one or more servo
motors to translate/rotate shift forks along a shift rail to select
a desired gear ratio. As generally understood by those of ordinary
skill in the art, an AMT may be used in applications with higher
torque requirements, for example.
[0023] As shown in the representative embodiment of FIG. 1, the
output shaft 36 is connected to a differential 40. The differential
40 drives a pair of wheels 42 via respective axles 44 connected to
the differential 40. The differential transmits approximately equal
torque to each wheel 42 while permitting slight speed differences
such as when the vehicle turns a corner. Different types of
differentials or similar devices may be used to distribute torque
from the powertrain to one or more wheels. In some applications,
torque distribution may vary depending on the particular operating
mode or condition, for example.
[0024] The powertrain 12 further includes an associated controller
50 such as a powertrain control unit (PCU). While illustrated as
one controller, the controller 50 may be part of a larger control
system and may be controlled by various other controllers
throughout the vehicle 10, such as a vehicle system controller
(VSC). It should therefore be understood that the powertrain
control unit 50 and one or more other controllers can collectively
be referred to as a "controller" that controls various actuators in
response to signals from various sensors to control functions such
as regenerative braking and downshifting, starting/stopping,
operating M/G 18 to provide wheel torque or charge battery 20,
select or schedule transmission shifts, etc. Controller 50 may
include a microprocessor or central processing unit (CPU) in
communication with various types of computer readable storage
devices or media. Computer readable storage devices or media may
include volatile and nonvolatile storage in read-only memory (ROM),
random-access memory (RAM), and keep-alive memory (KAM), for
example. KAM is a persistent or non-volatile memory that may be
used to store various operating variables while the CPU is powered
down. Computer-readable storage devices or media may be implemented
using any of a number of known memory devices such as PROMs
(programmable read-only memory), EPROMs (electrically PROM),
EEPROMs (electrically erasable PROM), flash memory, or any other
electric, magnetic, optical, or combination memory devices capable
of storing data, some of which represent executable instructions,
used by the controller in controlling the engine or vehicle.
[0025] The controller communicates with various engine/vehicle
sensors and actuators via an input/output (I/O) interface that may
be implemented as a single integrated interface that provides
various raw data or signal conditioning, processing, and/or
conversion, short-circuit protection, and the like. Alternatively,
one or more dedicated hardware or firmware chips may be used to
condition and process particular signals before being supplied to
the CPU. As generally illustrated in the representative embodiment
of FIG. 1, controller 50 may communicate signals to and/or from
engine 14, disconnect clutch 26, M/G 18, launch clutch 34,
transmission gearbox 24, and power electronics 56. Although not
explicitly illustrated, those of ordinary skill in the art will
recognize various functions or components that may be controlled by
controller 50 within each of the subsystems identified above.
Representative examples of parameters, systems, and/or components
that may be directly or indirectly actuated using control logic
executed by the controller include fuel injection timing, rate, and
duration, throttle valve position, spark plug ignition timing (for
spark-ignition engines), intake/exhaust valve timing and duration,
front-end accessory drive (FEAD) components such as an alternator,
air conditioning compressor, battery charging, regenerative
braking, M/G operation, clutch pressures for disconnect clutch 26,
launch clutch 34, and transmission gearbox 24, and the like.
Sensors communicating input through the I/O interface may be used
to indicate turbocharger boost pressure, crankshaft position (PIP),
engine rotational speed (RPM), wheel speeds (WS1, WS2), vehicle
speed (VSS), coolant temperature (ECT), intake manifold pressure
(MAP), accelerator pedal position (APPS), ignition switch position
(IGN), throttle valve position (TP), air temperature (TMP), exhaust
gas oxygen (EGO) or other exhaust gas component concentration or
presence, intake air flow (MAF), transmission gear, ratio, or mode,
transmission oil temperature (TOT), transmission turbine speed
(TS), torque converter bypass clutch 34 status (TCC), deceleration
or shift mode (MDE), for example.
[0026] Control logic or functions performed by controller 50 may be
represented by flow charts or similar diagrams in one or more
figures. These figures provide representative control strategies
and/or logic that may be implemented using one or more processing
strategies such as event-driven, interrupt-driven, multi-tasking,
multi-threading, and the like. As such, various steps or functions
illustrated may be performed in the sequence illustrated, in
parallel, or in some cases omitted. Although not always explicitly
illustrated, one of ordinary skill in the art will recognize that
one or more of the illustrated steps or functions may be repeatedly
performed depending upon the particular processing strategy being
used. Similarly, the order of processing is not necessarily
required to achieve the features and advantages described herein,
but is provided for ease of illustration and description. The
control logic may be implemented primarily in software executed by
a microprocessor-based vehicle, engine, and/or powertrain
controller, such as controller 50. Of course, the control logic may
be implemented in software, hardware, or a combination of software
and hardware in one or more controllers depending upon the
particular application. When implemented in software, the control
logic may be provided in one or more computer-readable storage
devices or media having stored data representing code or
instructions executed by a computer to control the vehicle or its
subsystems. The computer-readable storage devices or media may
include one or more of a number of known physical devices which
utilize electric, magnetic, and/or optical storage to keep
executable instructions and associated calibration information,
operating variables, and the like.
[0027] An accelerator pedal 52 is used by the driver of the vehicle
to provide a demanded torque, power, or drive command to propel the
vehicle. In general, depressing and releasing the pedal 52
generates an accelerator pedal position signal that may be
interpreted by the controller 50 as a demand for increased power or
decreased power, respectively. Based at least upon input from the
pedal, the controller 50 commands torque from the engine 14 and/or
the M/G 18. The controller 50 also controls the timing of gear
shifts within the gearbox 24, as well as engagement or
disengagement of the disconnect clutch 26 and the torque converter
bypass clutch 34. Like the disconnect clutch 26, the torque
converter bypass clutch 34 can be modulated across a range between
the engaged and disengaged positions. This produces a variable slip
in the torque converter 22 in addition to the variable slip
produced by the hydrodynamic coupling between the impeller and the
turbine. Alternatively, the torque converter bypass clutch 34 may
be operated as locked or open without using a modulated operating
mode depending on the particular application.
[0028] A brake pedal 53 is used by the driver of the vehicle 10 to
create a vehicle braking demand. Depressing brake pedal 53
generates a braking input signal that is interpreted by controller
50 as a command to decelerate the vehicle. To drive the vehicle
with the engine 14, the disconnect clutch 26 is at least partially
engaged to transfer at least a portion of the engine torque through
the disconnect clutch 26 to the M/G 18, and then from the M/G 18
through the torque converter 22 and gearbox 24. When the engine 14
alone provides the torque necessary to propel the vehicle, this
operation mode may be referred to as the "engine mode,"
"engine-only mode," or "mechanical mode." The M/G 18 may assist the
engine 14 by providing additional power to turn the shaft 30. This
operation mode may be referred to as a "hybrid mode," an
"engine-motor mode," or an "electric-assist mode."
[0029] To drive the vehicle with the M/G 18 as the sole power
source, the power flow remains the same except the disconnect
clutch 26 isolates the engine 14 from the remainder of the
powertrain 12. Combustion in the engine 14 may be disabled or
otherwise OFF during this time to conserve fuel. The traction
battery 20 transmits stored electrical energy through wiring 54 to
power electronics 56 that may include an inverter, for example. The
power electronics 56 convert DC voltage from the battery 20 into AC
voltage to be used by the M/G 18. The controller 50 commands the
power electronics 56 to convert voltage from the battery 20 to an
AC voltage provided to the M/G 18 to provide positive or negative
torque to the shaft 30. This operation mode may be referred to as
an "electric only mode," "EV (electric vehicle) mode," or "motor
mode."
[0030] In any mode of operation, the M/G 18 may act as a motor and
provide a driving force for the powertrain 12. Alternatively, the
M/G 18 may act as a generator and convert kinetic energy from the
powertrain 12 into electric energy to be stored in the battery 20.
The M/G 18 may act as a generator while the engine 14 is providing
propulsion power for the vehicle 10, for example. The M/G 18 may
additionally act as a generator during times of regenerative
braking in which rotational energy from spinning wheels 42 is
transferred back through the gearbox 24 and is converted into
electrical energy for storage in the battery 20. During a
regenerative-braking event, the transmission 24 may be downshifted
as the higher speed of transmission 24 allows for greater
regenerative braking power at higher efficiencies.
[0031] The electric motor (M/G) 18 usually operates at a constant
torque region when the motor speed is below a base speed, and
operates at a constant power region when the motor speed is above
the base speed. At this constant torque region, the electric motor
18 cannot deliver its maximum power. As a result, the braking power
recuperated may be limited by power of the electric motor 18 during
a braking event. To maximize regenerative braking efficiency, it is
often desirable to raise the motor speed to have it operate at the
constant power region to use the full potential of the electric
motor 18. At the instant when the brake pedal 53 is applied, the
electric motor speed is usually below the base speed, and the brake
torque increases from zero to a relatively constant level. It is
therefore important to have the first downshift of the automatic
transmission 24 scheduled at a proper time during the brake
application such that the constant torque region can be avoided to
maximize regenerative braking efficiency.
[0032] It should be understood that the schematic illustrated in
FIG. 1 is merely exemplary and is not intended to be limited. Other
configurations are contemplated that utilize selective engagement
of both an engine and a motor to transmit through the transmission.
For example, the M/G 18 may be offset from the crankshaft 28,
and/or the M/G 18 may be provided between the torque converter 22
and the gearbox 24. Other configurations are contemplated without
deviating from the scope of the present disclosure.
[0033] The MHT implementation is well-suited to provide
regenerative braking through the drivetrain, as discussed above.
Regenerative braking is a key feature utilized to increase vehicle
fuel economy. A regenerative braking event may begin with the
release of the accelerator pedal and application of the brake
pedal. During regenerative braking, the amount of negative torque
(or regenerative braking torque) that can be applied to the motor
is constrained by the minimum limit of the motor. To avoid a
situation where the motor operates in the constant torque region
where the motor cannot deliver its maximum power, it is important
to have the first downshift of the automatic transmission scheduled
at a proper time to maximize regenerative braking efficiency.
[0034] Embodiments according to the present disclosure provide
systems and methods of transmission downshift control for the time
period right after application of the brake pedal and prior to the
first regenerative braking downshift of the transmission during a
regenerative braking event. In particular, embodiments related to
controlling the first regenerative braking downshift using a
predicted brake pedal input rate and a predicted brake torque rate
are disclosed and described herein.
[0035] Now referring to FIG. 2, a flow diagram is shown having a
start 200 for classifying a predicted brake pedal input rate based
on road grade information. Road grade information can be obtained
from geographical information systems (GIS) and global positioning
systems (GPS) that can identify the location of a vehicle and the
surrounding terrain. The classification of the predicted brake
pedal input rate can be done with fuzzy logic rules or with lookup
tables. The predicted brake pedal input rate can be classified into
three levels: High, Medium, or Low, or it can be further refined
into additional levels.
[0036] With continual reference to FIG. 2, the control algorithm
may define a look ahead window or future time interval
(t.sub.start, t.sub.end) over which a predicted brake pedal input
rate is classified into input levels (e.g., High, Medium or Low),
as shown at step 202. In particular, the controller may receive
t.sub.start at step 202, which may be initiated by application of
the brake pedal. The look ahead window or future time interval
(t.sub.start, t.sub.end) may have a predefined length of time
t.sub.delta, resulting in t.sub.end=t.sub.start+t.sub.delta. In
step 204, the controller obtains the current or instantaneous road
grade value (Grade.sub.cur) and estimated road grade values within
the look ahead window (t.sub.start, t.sub.end). In step 206, the
controller may then calculate the average predicted road grade
(Grade.sub.prd) within the look ahead window (t.sub.start,
t.sub.end). In step 208, the controller then classifies the
predicted brake pedal input rate into an input level (e.g., High,
Medium or Low) based on a comparison of the current road grade
(Grade.sub.cur) and the average predicted road grade
(Grade.sub.prd). Specifically, the controller may determine a
difference between, or change in, the current road grade
(Grade.sub.cur) and the average predicted road grade
(Grade.sub.prd) to predict whether the brake pedal input rate will
be High, Medium, Low or some other refined level.
[0037] For example, a vehicle having a high elevation and traveling
on a steep decline would generally result in a higher brake pedal
input rate, which may be classified as High, as compared to a
vehicle having a low elevation and traveling on a gradual decline
that may have a lower brake pedal input rate classified as Low.
[0038] Moreover, a controller may use a driver's past history, if
available, to classify the predicted brake pedal input rate over
the look ahead window or future time interval (t.sub.start,
t.sub.end), as shown at step 210. Specifically, the controller may
determine a difference between the current road grade
(Grade.sub.cur) and the average predicted road grade
(Grade.sub.prd) and then refer to a lookup table to obtain a
driver's past history of brake input for that difference or change
in road grade. The controller can then use this information to
classify the predicted brake pedal input rate into three or more
input levels, such as High, Medium or Low, for example. One of
ordinary skill in the art would understand that classification of
the predicted brake pedal input rate is not limited to three input
levels and other classification methods could be implemented.
[0039] Now referring to FIG. 3, a flow diagram is shown having a
start 300 for classifying a predicted brake pedal input rate based
on vehicle headway range information. The headway range or distance
to a preceding vehicle may be measured using electromagnetic waves
and optics (e.g., LiDAR or RADAR). This calculation provides an
indication of the need to brake due to proximity to other vehicles.
A brake pedal input rate classification may be determined based on
this headway range.
[0040] With continual reference to FIG. 3, the control algorithm
may define a look ahead window or future time interval
(t.sub.start, t.sub.end) over which a predicted brake pedal input
rate is classified into an input level (e.g., High, Medium or Low),
as shown at step 302. In particular, the controller may receive
t.sub.start at step 302, which may be initiated by application of
the brake pedal. The look ahead window or future time interval
(t.sub.start, t.sub.end) may have a predefined length of time
t.sub.delta, resulting in t.sub.end=t.sub.start+t.sub.delta. In
step 304, the controller estimates the headway range and the rate
of change of the headway range to make a prediction. The range rate
may be predicted using a numerical method (e.g., moving average).
The range rate may be calculated over a period beginning with
depression of the brake pedal. At step 306, the controller may then
classify the brake pedal input rate based on the headway range and
predicted rate of change of the headway range over the look ahead
window (t.sub.start, t.sub.end). For example, a vehicle having a
small headway range and a large rate of change would require a
generally stronger brake pedal input rate than a vehicle having a
large headway range and small rate of change. In such a case, the
vehicle having the small headway range and large rate of change may
have a brake pedal input rate classification of High. Whereas, the
vehicle having a large headway range and small rate of change may
have a brake pedal input rate classification of Low.
[0041] Additionally, a controller may use a driver's past history,
if available, to classify the predicted brake pedal input rate over
the look ahead window or future time interval (t.sub.start,
t.sub.end), as shown at step 308. Specifically, the controller may
determine the predicted headway range and rate of change thereof
and then refer to a lookup table to obtain a driver's past history
of brake input for that headway range prediction. The controller
can then use this information to classify the predicted brake pedal
input rate into three or more input levels, such as High, Medium or
Low, for example. One of ordinary skill in the art would understand
that classification of the predicted brake pedal input rate is not
limited to three input levels and other classification methods
could be implemented.
[0042] Now referring to FIG. 4, a flow diagram is shown having a
start 400 for determining a predicted brake torque rate estimated
from a vehicle speed prediction within a look ahead window or
future time interval particular, the controller may receive
t.sub.start at step 402, which may be initiated by application of
the brake pedal. The look ahead window or future time interval
(t.sub.start, t.sub.end) may have a predefined length of time
t.sub.delta, resulting in t.sub.end=t.sub.start t.sub.delta. In
step 404, the controller obtains a vehicle speed prediction and
road grade prediction within the look ahead window (t.sub.start,
t.sub.end). As discussed above, road grade information can be
obtained from geographical information systems (GIS) and global
positioning systems (GPS) that can identify the location of a
vehicle and the surrounding terrain. A vehicle speed prediction and
braking torque prediction may be estimated using vehicle-to-vehicle
(V2V) or vehicle to infrastructure (V2I) communications, generally
referred to as V2X. The V2X prediction may include traffic flow
monitoring systems. The V2X method may use vehicle speeds and road
grade to estimate the braking torque. For example, a vehicle having
a high rate of speed and a negative road grade may require a higher
braking torque than a vehicle having a slow speed and a flat road
grade.
[0043] The vehicle speed within (t.sub.start, t.sub.end) may be
predicted using a linear representation. For example, if the
vehicle speed must reach zero within the distance between
t.sub.start and t.sub.end, the controller will anticipate a linear
slope to reach zero in the given distance. The vehicle speed may
also be predicted with a more complex method, which may include
other sources of information available to the vehicle controller,
as described above. At step 406, the instantaneous brake torque may
be estimated using known first principal equations when road grade,
the coefficient of road friction, and the coefficient of drag are
known. From this, the predicted brake torque rate can be determined
at step 408.
[0044] Now referring to FIG. 5, a flow diagram is shown having a
start 500 for determining a regenerative braking downshift torque
(Tq.sub.qdownshift), which is the torque value at which
regenerative braking downshift of the transmission is scheduled. At
step 502, the controller determines a minimum regenerative torque
(Tq.sub.regenMin) based on the minimum motor torque
(Tq.sub.motorMin) plus a safe margin (Tq.sub.safeMargin) associated
with safe operation of the motor taking into consideration motor
limits, resulting in Tq.sub.regenMin=Tq.sub.motorMin
Tg.sub.safeMargin. At step 504, the controller then estimates the
brake torque rate (Tqe.sub.brakeRate) or the brake pedal input rate
classification dependent on the vehicle information available. As
shown at steps 506 and 508, the regenerative braking downshift
torque (Tq.sub.downshift) is estimated based on the minimum
regenerative torque (Tq.sub.regenMin) adjusted by a shift time
(t.sub.shift) and the brake torque rate (Tqe.sub.brakeRate) or the
brake pedal input rate classification (f(Brake Rate
Classification)), dependent on available vehicle information. Here
the shift time (t.sub.shift) is the time between execution of the
transmission gear downshift and a change in the torque ratio due to
the shift.
[0045] If the brake torque rate (Tqe.sub.brakeRate) is obtained as
shown in step 506, the regenerative braking downshift torque
(Tq.sub.downshift) is calculated from the minimum regenerative
torque (Tq.sub.regenMin) plus the multiplication of the shift time
(t.sub.shift), the brake torque rate (Tqe.sub.brakeRate), and a
ratio (rt) that may be determined from the current motor speed and
brake torque rate (and which can be obtained by a lookup table
using such inputs). The resulting equation is then
Tq.sub.downshift=Tq.sub.regenMin+t.sub.shift*Tqe.sub.brakeRate*rt.
Alternatively, if the classification of the brake input rate is
available as shown at step 508, the regenerative braking downshift
torque (Tq.sub.downshift) is calculated from the minimum
regenerative torque (Tq.sub.regenMin) plus the multiplication of
the shift time (t.sub.shift) and a function of the brake rate
classification (f(Brake Rate Classification)). The resulting
equation is then
Tq.sub.downshift=Tq.sub.regenMin+t.sub.shift*f(Brake Rate
Classification). In the equations described above, it is assumed
that Tq.sub.motorMin is a negative number, Tq.sub.safeMargin is a
positive number, and Tqe.sub.brakeRate is a positive number.
[0046] Now referring to FIG. 6, a graphical illustration is
provided for showing the effects of downshift timing on motor
operation for a given brake torque request 603. In general, the
electric motor operates at a constant torque region when the motor
speed is below a base speed, and operates at a constant power
region when the motor speed is above the base speed. As discussed
above, the electric motor cannot deliver its maximum power when
operating at this constant torque region. And upon actuation of the
brake pedal, the electric motor speed is usually below this base
speed and the brake torque increases from zero to a relatively
constant level. It is therefore desired to raise the motor speed to
have it operate at the constant power region to use the full
potential of the electric motor to maximize regenerative braking
efficiency.
[0047] With continual reference to FIG. 6, it is shown that a later
gear downshift 600 leads to operation at this constant torque
region 606 where the motor is constrained from delivering its
maximum power, which results in less regenerative energy
capturedand therefore lower overall regenerative efficiency. In
contrast, FIG. 6 illustrates that an earlier downshift 602 allows
the motor to avoid operation at this constant torque region and
instead operate at a constant power region as shown at 604, and
therefore increases regenerative energy captured and overall
regenerative efficiency.
[0048] As can be seen by the representative embodiments described
herein, embodiments according to the present disclosure provide
robust and efficient transmission downshift control strategies for
improving regenerative braking efficiency and overall fuel
economy.
[0049] While exemplary embodiments are described above, it is not
intended that these embodiments describe all possible forms of the
disclosure. Rather, the words used in the specification are words
of description rather than limitation, and it is understood that
various changes may be made without departing from the spirit and
scope of the disclosure. Additionally, the features of various
implementing embodiments may be combined to form further
embodiments of the disclosure. While the best mode has been
described in detail, those familiar with the art will recognize
various alternative designs and embodiments within the scope of the
following claims. While various embodiments may have been described
as providing advantages or being preferred over other embodiments
with respect to one or more desired characteristics, as one skilled
in the art is aware, one or more characteristics may be compromised
to achieve desired system attributes, which depend on the specific
application and implementation. These attributes include, but are
not limited to: cost, strength, durability, life cycle cost,
marketability, appearance, packaging, size, serviceability, weight,
manufacturability, ease of assembly, etc. The embodiments discussed
herein that are described as less desirable than other embodiments
or prior art implementations with respect to one or more
characteristics are not outside the scope of the disclosure and may
be desirable for particular applications.
* * * * *